The IRS and Treasury Department blamed a human coding error that happened last year when Form 990-T began to be electronically filed. The nonpublic data was mistakenly included with the public data and all of it was available for searching and downloading on the agency’s website

An IRS research employee discovered the mistake in recent weeks, triggering a broader inquiry and leading to the removal of the data. As required under federal law governing major information-security incidents, the IRS notified Congress on Friday. Affected taxpayers will be notified in the coming weeks.

Last year, the news organization ProPublica published tax data about many of the wealthiest and highest-income Americans, triggering investigations. So far, it is unknown where that data came from.